The invention aims in particular to separate a structure into two parallel parts (or substructures). In the field of micro-electronics, it is often required to separate, within its thickness, a structure made up of stacked materials, at a bonding interface between two plates of materials bonded to each other, or in a plane of weakness, or in a plane of stresses existing in the stacked structure or at a deposition interface.
One known technique for separating bonded plates, represented in FIGS. 1 and 2, involves inserting a blade 1 by pushing it from the exterior of the bonded plates 2 and 3 at the level of the bonding interface 4 toward the interior of the latter interface and in its plane, in a radial direction. This leads to progressive opening of the bonded structure, a debonding front 5 being established in front of the blade, which therefore has a wedging role.
The blades used typically have a thickness of the order of one millimeter, possibly with a leading edge having a radius of curvature of the order of ten microns thick (or alternatively an angular leading edge as in FIG. 1). The thicker the blade, the wider the debonding area induced by introducing the blade and the further ahead of the blade the debonding front, because of the wedging effect.
Note that the area of the debonding zone induced by introducing the blade is inversely proportional to the bonding energy between the plates. Because of this, inserting a blade as represented in FIGS. 1 and 2 has been used to determine the bonding energy between plates.
There is also known a method of separating plates involving simultaneous insertion of a number of blades at different places at the periphery of the interface between the plates, each blade being directed toward the center of the stacked structure (see “Debonding of Wafer-Bonded Interfaces for Handling and Transfer Applications”, J. Bagdahn and M. Petzold, in Wafer Bonding Applications and Technology, published by Springer).
Separation of the bonded structure is therefore initiated from a number of peripheral zones, the central zone becoming debonded when the debonding fronts induced by the blades converge at it, for example.
Another known method for separating bonded plates, described in the document EP 1 385 683 B1, using a blade the leading edge of which is of concave shape, with a curvature corresponding to that of the periphery of the plates to be separated.
The movement of the blade is a movement in radial translation from the periphery toward the center of the structure to be separated, along the bonding interface, simultaneously bringing about separation of the plates over a large portion of the circumference of the interface.
In a variant of this latter method, represented in FIG. 3, as well as a concave blade 10, lateral blades 11 and 12 are applied to the structure, at a distance from the zone to which the concave blade 10 is applied, on either side thereof, and also in the plane of the bonding interface.
Each lateral blade 11 advances along a rectilinear trajectory along the flank of the plates, in the plane of the interface, in contact with the plates, and moving away from the zone opened by the concave blade 10, thus contributing to the separation of the two plates.
The above techniques have various drawbacks.
Firstly, inserting and advancing a blade causes deformation of the layers of material close to the separation zone. This deformation is critical in particular when, as represented in FIG. 4, one of the substructures, namely the thin substructure 2′, is thin compared to the thickness of the blade 1, in which case the debonding front is close to the blade 1 and the curvature R to which the thin substructure 2′ is subjected is such that it can break.
What is more, the blade used must not be too thin, given the forces to which it is subjected, which include compression forces when it is pushed and friction against the debonded surfaces of the plates. As a result a blade that is too thin manipulated in accordance with the known separation methods leads in practice to separation results that are not reproducible.
To this is added the possibility of damage to the separated surfaces caused by the introduction of impurities in the zone of movement of the blade.
Finally, in some cases, rebonding of the two separated substructures occurs after the blade has passed.
An object of the invention is to alleviate the aforementioned drawbacks by proposing a separation method and device that are well controlled even for separating structures including at least one thin substructure (for example less than 50 microns thick) to be separated over a large area.
To this end the invention proposes a method of separating a structure including a fragile zone delimiting in that structure two substructures to be separated, wherein at least one plane blade is advanced in a direction of advance in a separation plane corresponding to a median plane of the fragile zone, from an entry edge of the structure in the direction of an exit edge of the structure, so as to cause progressive separation of the two substructures, the method being characterized in that the inclination of the blade in the separation plane relative to the direction of advance is varied.
This method varies the distribution of the forces exerted on the structure by the blade. This improves the control of the separation process. The separation process being better controlled, a thinner blade can be used. This reduces the risk of damaging the surfaces laid bare by the separation and the risk of the substructures breaking. Finally, the method achieves separation of better quality, including when one of the substructures is thin.
According to an optional feature, the blade is advanced by applying forces to lateral portions of the blade, advantageously in traction.
This feature, novel in itself, can be implemented regardless of the movement of the blade, but it is of particular benefit in the context of applying a movement during which the inclination of the blade in the separation plane is modified as it advances.
Thanks to this feature, it is possible for there to be no thrust load on the blade as it advances between the two substructures.
It follows that it is not necessary for the blade to be very stiff, and so it can be thinner.
The method can therefore be used to separate structures including at least one thin substructure. According to an optional feature, the inclination of the blade is varied by causing it to turn about an instantaneous center of rotation situated in the separation plane (which point can be fixed or mobile relative to the blade).
According to an optional feature, the movement of the blade can include a component in translation in the separation plane perpendicular to the direction of advance.
According to an optional feature, a separation front delimiting within the retaining zone an open zone in which the two substructures are locally separated and a closed zone in which the two substructures are still joined, the inclination of the blade is varied in the separation plane to modify a curvature of said separation front.
According to an optional feature, a separation front delimiting within the retaining zone an open zone in which the two substructures are locally separated and a closed zone in which the two substructures are still joined, the inclination of the blade is varied in the separation plane to modify a length of said separation front, for example to increase it.
A length of the separation front can be taken between two lateral edges of the retaining zone.
According to one advantageous feature, the inclination is varied alternately in one direction and the other, as the blade advances.
According to an optional feature, the retaining zone or the fragile zone is a bonding interface produced by bonding a surface of a first of the two substructures and a surface of the second of the two substructures.
According to an optional feature, the retaining zone or the fragile zone is an internal zone of said structure mechanically weakened by a physical and/or chemical modification to which a constituent of said internal zone is subjected.
This can be a porous buried zone or a fragile buried zone created by implantation, for example implantation of gaseous species, or a buried film of lower viscosity, or a deposition interface.
According to an optional feature, one of the two substructures has a thickness less than 50 μm measured perpendicularly to said application plane.
According to an optional feature, one of the two substructures has a thickness less than 15 μm measured perpendicularly to said application plane.
According to optional features, the leading edge of the blade is linear, curved or angular.
According to an optional feature, the leading edge can be discontinuous. The blade can be serrated, for example, or consist of a number of elements, to which a movement is imparted as described above, the same or different for all the elements but with the same direction of advance.
According to an optional feature, the blade lies in its plane, parallel to the direction of advance, so that it remains disposed between the two substructures throughout the separation process.
In this way, the blade prevents rebonding by contact of the two substructures after the leading edge of the blade has passed.
According to an optional feature, a suction plate holds the structure in position by application of suction to a free surface of the structure.
This retains or immobilizes the substructure to which suction is applied during the separation operation, despite the forces applied by the blade.
According to an optional feature, the inclination movement of said blade is slaved to a signal from a video camera, for example an infrared video camera, filming the structure, for example in a direction transverse to the plane of separation.
Such infrared video cameras can be used to view the separation front. The rate of variation of the inclination of the blade in its plane and the advance of the ends of the blade can be slaved, for example to a separation front advance parameter.
According to an optional feature, the inclination of the blade is varied in a range of at least +/−20° relative to a reference inclination.
In practice, the inclination varies as a function of the structure and its constituent materials. It can take any value, for example 1° or even less, 10°, 30° or 90° or even more.
Furthermore, for the inclination variation, a lower rate of variation of inclination can be chosen if the structure consists of fragile materials. Likewise, a lower rate of variation of inclination can be chosen if the energy bonding the two structures together is high.
According to an optional feature, the structure comprises a semiconductor material.
The semiconductor material can be, for example: germanium, silicon, a silicon-germanium alloy SixGey, a III-V semiconductor compound, in particular GaAs, GaN or InP or a II-VI semiconductor compound such as ZnS, ZnSe or CdTe, or SiC.
According to an optional feature, the structure comprises an insulative material.
The insulative materials used can be glass, sapphire, lithium tantalate LiTaO3 or lithium niobate LiNbO3, diamond, garnet, alumina or polymers. A glass/glass structure is particularly preferable.
According to an optional feature, the structure comprises a conductive material.
A conductive material can be chosen from silicides, germanides or indium-tin oxide ITO, for example.
The material chosen can also be a metal, for example Cu, Ni, W, Pd or Pt.
According to an optional feature, the two substructures contain different materials.
A heterogeneous structure is advantageously used, for example, in particular an Si/glass structure, or Si/sapphire, SixGey/glass or germanide/glass structure, the retaining zone being a zone of bonding between the two materials.
According to an optional feature, one of the substructures has, in a direction parallel to the plane of the retaining zone or to the separation plane, from the edge toward the interior of the structure, a zone of maximum thickness and then a zone of minimum thickness significantly thinner than the zone of maximum thickness, the thickness being measured perpendicularly to the separation plane.
The thin zone, far from the edges of the structure, can result from thinning carried out beforehand perpendicularly to the plane of the retaining zone. The thick zone can have the original thickness of the substructure.
This enables a thin film of material to be separated from a structure to which the film was initially bonded, the film being stiffened by the presence of the thick zone, which can constitute a peripheral reinforcement around the film. This reduces the risk of the film breaking or becoming deformed.
According to an optional feature, the blade has a thickness at most equal to 100 μm.
The invention uses thin blades and therefore imposes only small deformations on the layers to be separated at the separation front. This reduces the risk of the separated structures breaking.
In practice at least the thickness of the blade transversely to the plane of the blade and/or the elasticity and/or the hardness of the blade is chosen as a function of the structure to be separated.
For example, a thickness of 80 μm is suitable for separating a 50 to 100 μm thick glass membrane from a 725 μm thick silicon substrate to which the membrane is bonded.
Moreover, the blade can be rigid or flexible, and in this case can take the form of a roll that can be unrolled, for example.
According to an optional feature, the blade comprises aluminum.
This material offers good corrosion resistance and good mechanical performance. In an advantageous variant, the blade, for example of aluminum, is covered with a film of Teflon™ facilitating introduction of the blade between the bonded structures and further preventing rebonding of the structures once debonded.
According to an optional feature, the blade comprises a plastic film or a thermoplastic polymer.
According to an optional feature, the blade comprises Kapton®.
According to an optional feature, the blade comprises paper.
According to an optional feature, the blade comprises a composite material such as carbon fiber.
Clearly, depending on the material chosen for the blade and the chosen thickness, a blade is obtained that can be described as rigid, flexible or semi-rigid.
The invention also provides a device for separating a structure including a fragile zone delimiting two substructures or a retaining zone between two substructures within said structure, the device including a plane blade and means for advancing the plane blade in a direction of advance in a separation plane corresponding to the median plane of the fragile zone or a plane parallel to a plane of the retaining zone, from an entry edge of the structure, to cause progressive separation of the two substructures, the device being characterized in that it includes means for varying the inclination of the blade in the separation plane relative to the direction of the advance.
According to an optional feature, said means for varying the inclination of the blade in the separation plane relative to the direction of advance vary said inclination in one direction and in the opposite direction.
According to an optional feature, means for advancing a plane blade in a direction of advance include a guide rail.
According to an optional feature, the blade has a leading edge between two ends, an oblong opening extending transversely to the direction of advance and situated in the vicinity of each end, said blade being connected to two parallel linear rails by rods engaged in the oblong openings.
The freedom of the rod to move in the oblong hole enables the blade to move in rotation in its plane or to move in translation in the lengthwise direction of the oblong hole.
According to an optional feature, the blade is connected to rails by arms of variable length.
According to an optional feature, the blade has a thickness at most equal to 100 μm.
According to an optional feature, the blade has a thickness at least equal to 50 μm.
According to an optional feature, the blade is flexible.
According to an optional feature, movement of said blade is caused by a motor winding up the blade at one end of the blade.
According to an optional feature, the blade comprises aluminum.
According to an optional feature, the blade comprises a plastic film.
According to an optional feature, the blade comprises Kapton®.
According to an optional feature, the blade comprises paper.
According to an optional feature, the blade comprises a composite material such as carbon fiber.
According to an optional feature, a leading edge of the blade is beveled.
This facilitates engaging the blade in the structure to be cut in the separation plane and minimizes the stress concentration near the separation front.
Alternatively, a leading edge of the blade can be rounded.
According to an optional feature, the device further includes a suction plate for retaining the structure by suction.
According to an optional feature, it further includes a video camera, for example an infrared video camera, for viewing the structure, for example in a plane perpendicular to the application plane.
The video camera is used to view the advance of the separation front between the two substructures.
According to an optional feature, the device further includes means for slaving the movement of the blade to a signal from the video camera.
Only the rotation component or the translation component of the movement can be slaved.
According to an optional feature, the suction plate is adjustable in height and in inclination.
This enables the structure to be separated to be moved to a position in which the blade is offered up to the retaining zone.